class MusicPlayer {
  var playlist List[String] = Nil      
  var currentSong String =           
  var playMode String = loop       

  def addSong(song String) Unit = {
    playlist = playlist + song        
  }

  def removeSong(song String) Unit = {
    playlist = playlist.filter(_ != song)  
  }

  def play() Unit = {
    currentSong = playlist.headOption.getOrElse()  
  }

  def next() Unit = {
 
    playMode match {
      case loop = loopPlayNext()
      case shuffle = shufflePlayNext()
      case single = singlePlayNext()
      case _ = println(Unknown play mode!)
    }
  }

  def loopPlayNext() Unit = {
    val index = playlist.indexOf(currentSong)  
    if (index == playlist.length - 1) {    
      currentSong = playlist.headOption.getOrElse()
    } else {                                 
      currentSong = playlist(index + 1)
    }
  }

  def shufflePlayNext() Unit = {

    currentSong = scala.util.Random.shuffle(playlist).headOption.getOrElse()
  }

  def singlePlayNext() Unit = {

    if (playlist.contains(currentSong)) {
      currentSong = currentSong
    } else {
      currentSong = playlist.headOption.getOrElse()
    }
  }
}